The present invention relates to fluorine-containing glass and, more particularly, is to provide a glass composition having improved transmittance in the UV region.
Synthetic silica glass articles have widely been used as optical parts and materials for optical transmission media such as optical fibers and optical wave guides, devices using various optical sources and working machines based on their excellent properties that they are transparent for radiation of a wide wavelength range covering UV region and vacuum UV region as well as near infrared region and visible region (which means they suffer less transmission loss), that they have enough resistance against change in temperature, and that they have a smaller linear expansion coefficient and an enough corrosion resistance. However, pure silica glass has a problem that they generate a new absorption band upon being irradiated with high energy UV rays, resulting in reduction in transmittance, change in refractive index and generation of fluorescent light.
In addition, in the use of semiconductor-manufacturing apparatus, LCD substrates and substrates for photomask, a much more improved heat resistance is being required for silica glass. For example, JP-A-5-97466 (Literature 1) (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes, as glass containing less contents of water and halogen and having a high heat resistance, dehydrated glass containing 90% by weight or more of silica, 1 ppm or less of aluminum, 100 ppm or less of water and 100 ppm or less of chlorine and having an annealing point of 1150xc2x0 C. or higher, and a process for producing the dehydrated glass utilizing microwave heating.
There has been fluorine-containing glass having a refractive index and a viscosity decreased by adding fluorine to pure quartz (synthetic silica glass), and it is known that the above-described problem with respect to UV transmittance can be solved by the addition of fluorine.
For example, JP-A-11-305419 (Literature 2) proposes synthetic silica glass containing 1000 ppm or more of OH group, 50 ppm or less of chlorine and 300 ppm or more of fluorine, and synthetic silica glass containing 100 ppm or less of OH group and 100 to 30000 ppm of fluorine, as glasses showing a high transmittance for radiation of a short wavelength region and a good UV ray resistance.
As one use of the synthetic silica glass under irradiation with UV rays, there is a use as a material for photomask substrate to be subjected to a photo-lithography process wherein an integrated circuit pattern is formed on a wafer upon manufacturing LSI. In recent years, with an increase in degree of integration and performance of LSIs, a technology of forming patterns with a finer line width of 0.2 xcexcm or less has been required, and thus light sources for exposure to be used in steppers for lithography, which are capable of emitting shorter wavelength radiation, having been intended to develop.
That is, it has been demanded that the light source be changed so as to provide shorter wavelength radiation, from conventional g-line emitted from a mercury lamp (436 nm in wavelength) to i-line (365 nm), KrF excimer laser (248 nm) and ArF excimer laser (193 nm), and further to a low-pressure mercury lamp (185 nm), ArCl excimer laser, ArCl excimer lamp (175 nm), Xe2 excimer laser and Xe2 excimer lamp (172 nm) and F2 laser (157.6 nm).
However, pure silica glass (SiO2) per se scarcely transmits F2 laser (157.6 nm in wavelength), though it can be used to the level of ArF excimer laser (193 nm). Even the synthetic silica glasses having an adjusted composition as described in the foregoing Literatures 1 and 2 are still insufficient as to UV ray transmittance and laser resistance in the vacuum UV ray region of 200 nm or less such as the wavelength of F2 laser radiation. Hence, it has been urged to develop synthetic silica glass having sufficient transmittance for UV rays with a much higher energy and UV ray resistance.
The present invention has been made in view of the foregoing problems.
Accordingly, an object of the present invention is to provide fluorine-containing silica glass which has a high UV ray-transmitting characteristic in a UV region of 200 nm or less in wavelength (vacuum UV region) such as a F2 laser radiation and has sufficient UV ray resistance.
Other objects and effects of the present invention will become apparent from the following description.
According to a recent report, xe2x80x9cDry and F doped Fused Silicaxe2x80x9d, so-called xe2x80x9cOH group-free, F-doped silica glassxe2x80x9d, is believed to be the best material as a material adapted for F2 laser radiation (xe2x80x9cConcerning photomasks adapted for F2 laserxe2x80x9d by Hiroki Jinbo, Optical Alliance, vol. 11, No. 4, pp. 20 to 25, published by Nikkan Kogyo Shuppan in year 2000: Literature 3). However, investigation of the present inventors revealed that, even the concentration of OH group is at a low level the glass fails to have sufficient transmittance and laser resistance when the concentration of fluorine is at a low level.
As a result of extensive investigation on glass compositions, the inventors found that transmittance properties in a vacuum UV region of 200 nm or less and laser resistance can be improved by adjusting contents and content ratio to such specific ranges that OH group content is 10 ppm or less, Cl content is 10 ppm or less, and F content is 1000 ppm or more, with F/Cl ratio being 100 or more. The present invention is based on this finding.
That is, the above-described objects of the present invention have been achieved by providing the following fluorine-containing glasses.
(1) Fluorine-containing glass which comprises silica and contains, in said silica, not more than 10 ppm of OH group, not more than 10 ppm of Cl and not less than 1000 ppm of F, said fluorine-containing glass having a concentration ratio of F/Cl of 100 or more.
(2) Fluorine-containing glass which contains not more than 10 ppm of OH group, not more than 10 ppm of Cl and not less than 1000 ppm of F, and has a concentration ratio of F/Cl of 1000 or more.
(3) Fluorine-containing glass which contains not more than 10 ppm of OH group, not more than 1 ppm of Cl and not less than 1000 ppm of F, and has a concentration ratio of F/Cl of 10000 or more.
The reasons why transmittance properties in the vacuum UV region and laser resistance can be improved by the constitution of the invention may be considered as follows.
It is considered that the structure of the fluorine-containing glass is fundamentally based on SiO1.5F structure (3SiO2+SiF4xe2x86x924SiO1.5F). That is, the fluorine-containing glass is considered to have the structure wherein Si is partly ended with F.
In order to ensure 80% or more transmittance at wavelengths in the vacuum UV region, particularly at 157 nm which is the wavelength of excimer laser and to obtain enough resistance against a F2 laser, it is considered to be necessary to sufficiently replace the bonds such as Sixe2x80x94OH, Sixe2x80x94Cl and Sixe2x80x94Si by Sixe2x80x94F bond having a higher bond energy.
In the invention, portions of glass ended with F (Sixe2x80x94F) are increased by decreasing the Cl content, increasing the F content, and adjusting the concentration ratio of F/Cl to 100 or higher, thus transmittance in the UV region and laser resistance being improved.